Lithium secondary batteries having high energy density which are used for various applications such as cellular phones, digital cameras, laptop computers, mobile digital media players and MP3 players use liquid electrolytes generally. Therefore they have explosion risk and thus complicated layouts for preventing explosion of batteries are needed.
Since solid electrolytes including polymers can solve the above-mentioned problem and make it possible to develop small scale and thin layer type batteries, studies on this field have been carried out actively. However, there are some problems for commercializing the solid electrolytes, since they have significantly lower ionic conductivity than liquid electrolytes. Therefore, the development of a material for electrolytes sustaining solid phase and having higher ionic conductivity is urgently required.
The polymer material has been used for a long period of time as solid phase electrolytes is polyethylene oxide (PEO) such as polyethylene glycol and it has ionic conductivity like liquids, although it is a solid phase material. Alkali metal salts in the polymer may exist as stable ions since cations dissociated from the salts form a coordination complex through coordinate bonds with oxygen atoms in the polymer and thus can be stabilized thereby. These ions can move through the polymer as dissociated state and thus show ionic conductivity. However, linear polyethylene oxide has somewhat broad crystallizing area and low ionic conductivity of 10−8 S/cm at room temperature thereby. The characteristic acts as a bottleneck for commercializing PEO as solid electrolytes.
Thus, over the past 20 years, various studies to enhance ionic conductivity of PEO by lowering crystallinity thereof have been made. For representative examples, a method of blending with a non-crystallized polymer (Kulasekarapandian et al., Int. J. Eng. Res. Dev., 5(11): 30-39, 2013), a method of enhancing flexibility of polymer chain by adding a plasticizer (Korean Patent No. 1368870), a method of adding side chains of low molecular weight ethylene oxide to main polymer chains (Korean Patent No. 1351846), a method of fixing low molecular weight polyethylene oxide to polymer having crosslinked network structure (U.S. Pat. No. 5,240,791), and a method of blending the PEO with inorganic nano-filler (Sharma et al., Open Journal of Organic Polymer Materials, 2: 38-44, 2012) were reported.